Processability is the ability to economically process and shape a polymer uniformly. Processability involves such elements as how easily the polymer flows, melt strength, and whether or not the extrudate is distortion free. Typical metallocene catalyzed polyethylenes (mPE) are somewhat more difficult to process than low-density polyethylenes (LDPE) made in a high-pressure polymerization process. Generally, mPEs require more motor power and produce higher extruder pressures to match the extrusion rate of LDPEs. Typical mPEs also have lower melt strength which, for example, adversely affects bubble stability during blown film extrusion, and they are prone to melt fracture at commercial shear rates. On the other hand, however, mPEs exhibit superior physical properties as compared to LDPEs.
It is not unusual in the industry to add various levels of an LDPE to an mPE to increase melt strength, to increase shear sensitivity, i.e., to increase flow at commercial shear rates; and to reduce the tendency to melt fracture. However, these blends generally have poor mechanical properties as compared with neat mPE.
Traditionally, metallocene catalysts produce polymers having a narrow molecular weight distribution. Narrow molecular weight distribution polymers tend to be more difficult to process. The broader the polymer molecular weight distribution the easier the polymer is to process. A technique to improve the processability of mPEs is to broaden the products' molecular weight distribution (MWD) by blending two or more mPEs with significantly different molecular weights, or by changing to a polymerization catalyst or mixture of catalysts that produce broad MWD polymers.
In the art specific metallocene catalyst compound characteristics have been shown to produce polymers that are easier to process. For example, U.S. Pat. No. 5,281,679 discusses metallocene catalyst compounds where the ligand is substituted with a substituent having a secondary or tertiary carbon atom to produce broader molecular weight distribution polymers. U.S. Pat. No. 5,470,811 describes the use of a mixture of metallocene catalysts for producing easy processing polymers. U.S. Pat. No. 5,798,427 also addresses the production of polymers having enhanced processability using a metallocene catalyst compound where the ligands are specifically substituted indenyl ligands.
U.S. Pat. No. 6,339,134 (Crowther et al.) and U.S. Pat. No. 6,388,115 (Crowther et al.), describe a metallocene catalyst compound represented by the formula LALBMQn, where MQn may be, among other things, zirconium dichloride, and LA and LB may be, among other things, open, acyclic, or fused ring(s) or ring system(s) such as unsubstituted or substituted, cyclopentadienyl ligands or cyclopentadienyl-type ligands, heteroatom substituted and/or heteroatom containing cyclopentadienyl-type ligands. The Q ligands include hydrocarbyl radicals having from 1 to 20 carbon atoms.
PCT Publication No. WO 03/064433 (“Holtcamp”), relates to polymerization catalyst activator compounds that are either neutral or ionic and include a Group 13 atom, preferably boron or aluminum, bonded to at least one halogenated or partially halogenated heterocyclic ligand. The publication states that such activator compounds may be used to activate metallocene catalyst compositions. One such catalyst composition is cyclotetramethylenesilyl(tetramethyl cyclopentadienyl) (cyclopentadienyl)zirconium dimethyl(“(C4H8) Si(C5Me4)(C5H4)ZrMe2”).